The present disclosure relates to an optical waveguide element.
An optical waveguide element using a planar light wave circuit (PLC) formed of quartz-based glass is used in an optical device using an optical communication or the like. In the optical waveguide constituting the PLC, a technique of using zirconia (ZrO2) as a dopant for increasing a refractive index is disclosed (see JP 2013-210623 A). ZrO2 is a material having a higher refractive index and a smaller thermal expansion coefficient than germania (GeO2). When ZrO2 is used as a dopant, a relative refractive index difference Δ of a core (an optical waveguide) to a cladding portion (hereinafter, simply referred to as a relative refractive index difference Δ) can be largely increased as compared with the PLC using GeO2 as a dopant. Accordingly, since a minimum bending radius allowed for the optical waveguide decreases, a decrease in size, a decrease in cost, and a high density integration of the PLC can be expected. For that reason, ZrO2 is expected as a material capable of reducing a stress remaining in the optical waveguide while realizing a decrease in size of the PLC, the optical waveguide element using the same, and the optical device.
Further, as the optical waveguide element of which the relative refractive index difference Δ is high, there is known an optical waveguide element including a silicon thin wire optical waveguide or an optical waveguide formed of an InP-based semiconductor material (for example, GaInAsP) other than PLC.
Incidentally, in the optical waveguide element, there is a case in which a slit is formed in the cladding portion so as to divide a certain optical waveguide into two parts and an optical filter is inserted into the slit. For example, when TE polarized light propagates through the optical waveguide (light of a linear polarized wave having a polarization direction parallel to a principal surface of a substrate provided with the cladding portion) is converted into TM polarized light (light of a linear polarized wave having a polarization direction orthogonal to the TE polarized light), a configuration of inserting a half wavelength plate into the slit is adopted in some cases. At this time, the half wavelength plate is installed inside the slit so that an optical axis (a high-speed axis or a low-speed axis) forms an angle of 45° with respect to a principal surface of the substrate. In the optical waveguide element with such a configuration, when the TE polarized light propagates through one optical waveguide of the divided optical waveguides into the half wavelength plate, the polarization direction rotates by 90° so that the light becomes the TM polarized light by the half wavelength plate and is input into the other optical waveguide of the divided optical waveguides. Similarly, when the TM polarized light propagates through the one optical waveguide into the half wavelength plate, the light becomes the TE polarized light by the half wavelength plate and is output to the other optical waveguide. Further, when the TM polarized light propagates through the other optical waveguide into the half wavelength plate, the light becomes the TE polarized light by the half wavelength plate and is input to the one optical waveguide.